The present invention is directed to hollow stainless steel shafts or tubing, assemblies and methods of making same. More particularly, the present invention is directed to brazed, air hardenable stainless steel tubular shafts. In addition, the present invention is directed to reinforced stainless steel tubular shafts, the reinforcement member being brazed inside the tubular shaft. The present invention is also directed to producing unitary assemblies of air hardenable stainless steel tubing which may be reinforced at pre-selected locations.
The shafts or tubing of the present invention are brazed in a controlled atmosphere furnace. Brazing in a controlled atmosphere furnace allows for the hardening and brazing of air hardenable stainless steel in the same brazing temperature cycle. The shafts produced by this process are light, flexible and strong. In addition, the process of the present invention allows for the reinforcement of air hardenable hollow stainless steel shafts with stainless steel inserts of any pre-selected dimension. The inserts are brazed inside the hollow shafts, thereby providing tubular shafts with reinforcement at precise locations.
Hollow metallic shafts are ubiquitous in our society. They are used in products where lightness and strength are required. Products which require hollow shafts include esoteric goods, such as sporting goods, and utilitarian ones e.g. airplane fuselage supports, cantilever street light poles, ladders, furniture, tools, etc. With respect to sporting goods, hollow shafts are used for golf clubs, bicycle frames, ski poles, fishing poles and the vertical supporting members of volleyball nets. In addition to lightness and strength, hollow shafts used in sporting goods sometimes require precise flexibility. Golf clubs and bicycle frames also require torque resistance.
Materials presently used to construct hollow metallic shafts include aluminum, low and high carbon steels, coated steels, alloy steels and composites. These materials may be objectionable for numerous reasons, including, but not limited to, weight, weakness, rigidity, flexibility, torque resistance and price. A hollow shaft which is light, strong, flexible and inexpensive is needed.
With respect to hollow shafts presently used in sporting goods, these shafts comprise fiberglass, kevlar, metals, graphite and composites. While fiberglass, composite and graphite shafts are light and strong, their flexibility can be limited. This characteristic results in their snapping under loads in which a metal shaft would not. As for hollow metallic shafts, aluminum is light, but lacks the strength of steel. Titanium and formerly classified metals generally have very favorable characteristics, but are in limited supply and are therefore very expensive. Carbon steels are strong but oxidize easily. Prior art carbon and alloy steel tubular shafts are often inadequate because of their weight and their tendency to corrode.
In addition to the difficulties of weight and corrosion, welding, the joining technique of choice for metallic hollow shafts, weakens practically all metals adjacent the weld. Even the welding of austenitic stainless steel introduces impurities into the grain boundaries which often result in oxidation and failure of the weld. Since corrosion resistance, as well as lightness, strength and flexibility, is a desirable characteristic for hollow shafts, graphite and composites have become the materials of choice.
Despite the limitations of stainless steel to provide a completely corrosion resistant hollow shaft, it has been, and still is, used to manufacture hollow shafts. Austenitic stainless, as opposed to ferritic and martensitic, is the most widely used type of stainless steel for producing tubular shafts. Austenitic stainless is non-magnetic and is not air hardenable. Of the stainless steels, only martensitic is air hardenable.
In addition to austenitic, that is, 300 series, stainless steel's limitations vis-a-vis corrosion resistance, stainless steel cannot presently be fashioned to provide strength where needed at a competitive cost utilizing prior art methods. Austenitic steel can be drawn so it is butted and thereby provide reinforcement, however drawing requires a significant amount of labor, thereby increasing the cost. In addition, excessive drawing will introduce impurities into the grain boundaries of austenitic stainless steel.